walk together, we talked about the relationship between the nature of a building, primarily its height, and the soil on which it sits. the slide is a good reminder of that. >> the ground has a natural period. the denser the material, the harder the material, the shorter the period of the ground. bay mud, which is present close to the shoreline, and the green area, the crown has a very long period. when the earthquake comes, there is a link of time that it takes the ground to go from one, slosh back and forth, measured in seconds. the deeper the bay mud deposits, the greater the length of time it takes for the motion to go one direction and back. so as you approach soft sites, loose sand sites, bay mud sites, the period of the site is long.